1. Field of the Invention
The present invention relates to drive control for a laser.
2. Description of Related Art
Lasers are used as the light source in optical disc drives such as commonly used for auxiliary storage in computer systems. Generally speaking, individual laser elements differ greatly in their characteristics, and the relationship between input current and output light power is not constant as a result of temperature change and aging of the laser element. Conventional optical disc drives therefore maintain a desired laser power using feedback power control to control output light power to a desired level while monitoring the emitted power. With a recordable optical disc drive the output power must be controlled while emitting the laser beam in pulses according to the data to be recorded (hereinafter referred to as xe2x80x9crecord dataxe2x80x9d), and various methods of accomplishing this have been proposed.
Broadly speaking, prior art methods for controlling the output power of a pulse-emitting laser can be grouped in two categories. The first category determines and remembers the current required for pulse emissions by emitting test beams when data is not being recorded, and then uses the stored current value to drive the laser when recording data. This is referred to as the test emission method. In the second category, a high speed sample hold circuit extracts a period in the record data where the laser power is locally constant, and discretely controls power during recording. This is referred to as the sample-hold method. This sample-hold method is more fully described in Japanese Patent Laid-open Publication (kokai) H09-171631.
Problems with the above two prior art methods are described below.
Although the test emission method determines the drive current, laser temperature gradually rises when data is recorded continuously for an extended time, and even if the current is held constant at the stored test current level, emission power gradually changes due to the change in laser temperature. To resolve this problem, a track format having an area (referred to as a xe2x80x9cgapxe2x80x9d) for test emission disposed at regular intervals in the recording tracks is used, and the change in output power is suppressed to a negligible level by repeating the test emission at a regular time interval. An obvious drawback to this is that the area usable for data recording is reduced by the area of the gaps, and the storage efficiency of the recording medium thus drops.
With the sample hold method, the frequency characteristic of the emission strength monitor may not be sufficient when the frequency of the record data is increased in order to improve recording speed. In addition, extremely high response performance is required in the high speed sample hold circuit in this case, and this invites a cost increase due to the parts that must be used.
To resolve the above problems an object of the present invention is to constantly, continuously control the output power of a pulse-emitting laser to a desired level while recording data in an optical disc drive, and to so control the laser without using test emissions or a high speed sample hold circuit.
To achieve this object, a laser drive method according to the present invention comprises detecting beam emission power from a light source and generating a monitor wave; receiving data; generating an expected wave for the beam power based on the received data; calculating a waveform difference between the generated monitor wave and expected wave; controlling current flow from the bias current source based on the calculated waveform difference; and emitting a beam from the light source based on the controlled current flow of the bias current source.
Yet further preferably, the laser drive method further comprises detecting and outputting as a monitor amplitude a peak-to-bottom difference of the bandwidth-limited monitor wave; detecting and outputting as an expected amplitude a peak-to-bottom difference of the bandwidth-limited expected wave; determining an amplitude difference of the output monitor amplitude and the output expected amplitude; and adjusting the current flow of the pulse current source based on the determined amplitude difference.
To further achieve the above objects, a laser drive system according to the present invention has an emission power monitoring unit which detects beam emission power from a light source and generating a monitor wave; an expected waveform generating unit which receives data and based on the received data generating an expected wave for the beam power; a differential operator which calculates a waveform difference between the monitor wave generated by the emission power monitoring unit and the expected wave generated by the expected waveform generating unit; a bias current source which controls current flow based on the waveform difference calculated by the differential operator. The laser drive system then emits a beam from the light source based on the current flow controlled by the bias current source.
Yet further preferably, the laser drive system additionally has a monitor amplitude detecting unit which detects and outputs as a monitor amplitude the peak-to-bottom difference of the bandwidth-limited monitor wave; an expected amplitude detecting unit which detects and outputs as an expected amplitude the peak-to-bottom difference of the bandwidth-limited expected wave; an amplitude differential operator which determines an amplitude difference between the monitor amplitude output from the monitor amplitude detecting unit and the expected amplitude output from the expected amplitude detecting unit. In this case, the pulse current source adjusts the current flow based on the amplitude difference determined by the amplitude differential operator.
The laser drive method and laser drive system according to the present invention can constantly and continuously control the power of a pulse-emitting laser without using test emission feedback or a high speed sample hold circuit. The laser drive method and laser drive system of the present invention can therefore be used in an optical disc drive to achieve an extremely high data recording rate and recording efficiency.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.